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Stock Problems/IssuesDiscussions related to warranty related issues and repairs, or other problems with the OEM parts and software for MINI Cooper (R50), Cabrio (R52), and Cooper S (R53) MINIs.
Then buy the ones sold by Canton or other brands, which are a true pressure caps. The stock ones on the Mini has a built-in safety, but the system is made to be pressurized. Drilling a hole in the stock cap won't do much except allowing coolant to leak. I have a Forge tank with the stock cap and no problems, go figure.
I've been running the forge since before the "recall" on them, about 2003. No problems at all. Replaced the cap once (stock cap) around 300k miles. I'd say go aluminum.